Aquaponics: Culling the Older Fish

Our aquaponics system is now approaching its fourth year since we installed it here at Ecofilms and over the years a multitude of vegetables and fish have gone through it.

Countless tomatoes, cucumbers, herbs, lettuce and fish have been grown from seed and fingerling, right out to maturity to ultimately grace our dinner plate. In fact we’ve kept some of the older fish, just to see how they would fare. Like an old friend, this 2lb barramundi (pictured) grew accustomed to being fed and would eagerly hang out near the entry to the hatch to be the first to feed on the fish pellets each morning.

Whilst the younger fish would slap the water with vigor and determined zeal, this older fish grew relaxed over the year and would slurp the feed down with a quiet measured grace .

Sadly though the time has come to keep the system in balance and the water quality clean so it meant a few of the old timers days were numbered.

Its sad in a way because you do get attached to your fish a bit and like to see them swim about majestically. They hardly know their number is up when D-day approaches.

Out comes the net. The fish is not bothered or stressed at all as it is edged inside and then its lifted out and quickly dispatched humanely. In fact there is very little stress and struggle for the fish. This barramundi just swam right into the net and was lifted right out of the tank in seconds. Catching fish in your backyard never looked so easy!

Most people find it incredible to be able to grow fish in the backyard like this. For many its the reason they become interested in Aquaponics in the first place.

But growing fish is not the reason we do it at all. The fish are the nutrient providers. They are the motors that drive the system. That power the feed for the vegetables. The problem is when we have too many “motors” powering the system, that it can get out of balance, providing too much nutrient and turning a valuable resource into a pollutant.

Hence the need to cull some of the older fish and bring the system back into balance. Fishing is never easier than this, but you have to harvest the fish and eat them or give them away. Its a wonderful way to have the BBQ blazing away and the aquaponics tank nearby and selecting your fish is easy as pie. We’ve had catfish, jade perch and barramundi grown in our system and all live quite happily together and all eat the same feed.

So what do the fish taste like? Do they have any off flavours? Not at all. They taste like – well, just like fish you’d buy in the local store. Even better because they are fresh and clean. No chemicals or dangerous metals in their flesh. Clean wholesome food grown at home gives you a great feeling of success too.

Barramundi have a reputation as a fine dining fish, but we prefer Jade Perch. Its a slightly oilier fish and tends to have a stronger flavour. It all comes down to personal preferences.

How hard is it to keep fish in an Aquaponics system?

Over the four years, we’ve found the system seems to mature with little effort needed on our part to maintain the system.

You do need to feed the fish regularly but if they miss out for a few days, they can survive quite nicely without much effort. A mature aquaponics system only needs to be inspected occasionally.

I give the pump a clean once a fortnight for any obstruction to the water intake vents. The sides of the tank have a slime coating now. Yes, there is a thin coating of algae on the sides of the tank. I give it a clean once or twice a year, seeing it more like a protective natural barrier rather than an air depleting system, because algae will take some precious oxygen out of your water, but we haven’t seen the fish suffer for lack of air..

Clean water well filtered is probably the main priority with enough oxygen to keep the fish alive. Keeping a standby pump in the garage is also a good idea but we are still using our first 60 Watt pump and it hasn’t missed a beat.

A separate 8 Watt four-outlet aquarium air pump will help supplement the needs to the fish quite well. We’ve only replaced that unit once, mainly because the older pump seemed to vibrate and make a loud noise like its little internal bellows were running ragged.

The only other thing to do once a fortnight is to check the pH and adjust mainly with hydrated lime the tendency of the system to gradually edge to the more acidic side of things. Its pretty much a no-brainer once you get used to balancing out the system. Other than adequate access to reliable power to keep an aquaponics system running, you can grow out some very nice fish. We’d like to try trout next but given our warm temperatures, it would have to be a very short season to manage trout in a semi tropical environment.

So whats on the menu tonight?

Spicy Barbecued Barramundi

Preheat barbecue for high heat.

In a small bowl stir together a large knob of crushed garlic, finely chopped onion, a sprinkle of paprika, a squeeze of lemon and sea salt. Sprinkle seasonings onto the fish.

In a small saucepan over medium heat melt the butter with the garlic and parsley. Remove from heat when the butter has melted and set aside.